The present invention relates to an optical communications system and an optical cross connector apparatus suited for use in the optical communications system.
One of conventional optical communications systems is disclosed in the form of an optical communications network which comprises a plurality of nodes connected to each other by optical fibers for transmission of wavelength multiplexed light, each the node having an optical cross connector for switching the path of light signal according to its wavelength as stated in Appendix 1 (0 plus E, No. 194, pp. 75-79, and pp. 100-105, including FIG. 3 on p. 77).
In such a conventional optical communications network, different wavelengths of light are split to pass their respective paths. For example, a wavelength lambda n of the light is destined to a particular node n in the optical communications network.
For developing a large scale optical communications system using a plurality of the conventional optical communications networks, it is necessary to increase the number of nodes and the number of wavelengths to be multiplexed. When the number of wavelengths to be multiplexed is increased, the implementation of relevant components or arrangement has to be modified.
It is known that the number of wavelengths to be multiplexed is substantially 32 at maximum in the latest technology. The more the number of wavelengths to be multiplexed, the more the number of light sources and wavelength filters are needed for optical signal processing. Also, an arrangement of the nodes becomes troublesome. With the use of the principle of such conventional optical communications networks, the development of a large scale optical communications system will thus be critical.
The foregoing drawbacks about the number of wavelengths to be multiplexed and the arrangement of the system are common to not only wavelength multiplexing but also other multiplexing methods including packet multiplexing and subcarrier multiplexing. They may also be accounted when the nodes are connected by bundles of optical fibers in a space multiplexing communications system.
It may be probable to develop a fair-scaled optical communications system with the conventional optical communications networks where both the number of nodes and the number of wavelengths to be multiplexed are increased. However, increasing the number of nodes and the number of wavelengths to be multiplexed will hardly contribute to the flexibility of changing a topology in the optical communications network.
Each node in the conventional optical communications network includes an optical cross connector for switching the path of light depending on its wavelength. Such a traditional optical cross connector is disclosed in Appendix 2, "Journal of lightwave technology", Vol. 14, No. 6, pp. 1410-1421, July in 1996.
The traditional optical cross connector (See FIG. 9 on p. 1417 of Appendix 2) comprises a plurality of units. The action of the traditional optical cross connector includes: 1. separating a wavelength multiplexed light signal (referred to as wavelength multiplexed optical signal) received by the input port of a unit into wavelengths (or wavelength channels) with a demultiplexer element; 2. directing each wavelength of the light signal to a first corresponding intermediate port; 3. combining a number of wavelengths of the light signal supplied from their respective first intermediate ports with a first star coupler to have a composite light signal which is delivered to a second intermediate port; 4. directing the composite light signal produced by the first star coupler to a second star coupler provided corresponding to the second intermediate ports for further combining; and 5. releasing a resultant combined light signal from an output port allocated to each the second star coupler. Accordingly, each wavelength of the light signal received by the input port of each unit can successfully be distributed to a desired one of the output ports.
In the traditional optical cross connector disclosed in Appendix 2, the light signal runs across the two, first and second, start couplers while being transferred from the input port to the output port. It is known that loss of the light signal in the star coupler is not negligible. The more the number of the input ports of the star coupler or the number of light signal components to be combined by the star coupler, the lower the intensity of a light signal output will be released from the output of the star coupler. When the number of wavelength channels is increased, the number of the first intermediate ports in the traditional optical cross connector has to be increased. Hence, the number of the inputs of the first and the second star coupler will be increased. Also, the higher the number of the input ports or the number of the units, the higher the number of the inputs of the second star coupler is needed. As the result, increasing the number of the wavelength channels or the input ports will decline the intensity of a light signal output.
The light signal released from the output port is a wavelength multiplexed signal produced by the second star coupler. The second star coupler is however liable to add the light signal output with a crosstalk generated by an optical switch provided at the preceding stage of the first star coupler. It is thus difficult in the traditional cross connector to prevent declination of the quality of the signal output due to the crosstalk over a wavelength. For compensation, the traditional optical cross connector is provided with a particular type of the optical switches having a higher optical quenching ratio (which can be fabricated and operated only with much difficulty).